I'm studying MUI, and in the course, the instructor asks me to style just one component and not the entire theme.

For that, it uses the makeStyles function and spreads the theme.mixins.toolbar. The problem is when I do this, I have the following error:

TypeError: Cannot read properties of undefined (reading 'toolbar')

This course is apparently in version 4, and I am using version 5. Despite this, my code appears to follow the changes that the documentations asks for. So what could be causing this error?

app.js

import "./App.css";

import Header from "./components/ui/Header";

import { ThemeProvider } from "@material-ui/core/styles";

import theme from "./components/ui/Theme";



function App() {

    return (

        <ThemeProvider theme={theme}>

            <Header />

        </ThemeProvider>

    );

}



export default App;

Theme.js

import { createTheme } from "@material-ui/core/styles";



const arcBlue = "#0B72B9";

const arcOrange = "#FFBA60";



export default createTheme({

    typography: {

        h3: {

            fontWeight: 100,

        },

    },

    palette: {

        common: {

            blue: `${arcBlue}`,

            orange: `${arcOrange}`,

        },

        primary: {

            main: `${arcBlue}`,

        },

        secondary: {

            main: `${arcOrange}`,

        },

    },

});

header/index.jsx

import React from "react";

import AppBar from "@mui/material/AppBar";

import Toolbar from "@mui/material/Toolbar";

import useScrollTrigger from "@mui/material/useScrollTrigger";

import Typography from "@mui/material/Typography";

import { makeStyles } from "@material-ui/styles";



function ElevationScroll(props) {

    const { children, window } = props;

    const trigger = useScrollTrigger({

        disableHysteresis: true,

        threshold: 0,

        target: window ? window() : undefined,

    });



    return React.cloneElement(children, {

        elevation: trigger ? 10 : 0,

    });

}



const useStyles = makeStyles((theme) => ({

    toolbarMargin: { ...theme.mixins.toolbar },

}));



function Header() {

    const classes = useStyles();

    return (

        <React.Fragment>

            <ElevationScroll>

                <AppBar color="primary">

                    <Toolbar>

                        <Typography variant="h3" component="h3">

                            Nome de teste

                        </Typography>

                    </Toolbar>

                </AppBar>

            </ElevationScroll>

            <div className={classes.toolBarMargin} />

        </React.Fragment>

    );

}



export default Header;